In an engine braking method, an exhaust valve is opened at a later stage of a compression stroke of an engine piston and closed at an earlier stage of an expansion stroke (generally before the normal opening of the exhaust valve). An example of an engine brake was disclosed by Cummins in the disclosure of U.S. Pat. No. 3,220,392 in 1965. In the brake system, a mechanical input is transmitted to an exhaust valve to be opened through a hydraulic circuit. The hydraulic circuit generally includes a brake piston reciprocating in a brake piston bore. The reciprocating motion comes from the mechanical input of the engine, such as the motion of the engine's fuel injection cam or the neighboring exhaust cam. The motion of the brake piston is transmitted through hydraulic fluid to a slave piston located in the hydraulic circuit, causing the slave piston to reciprocate in a slave piston bore. The slave piston acts, directly or indirectly, on the exhaust valve, thereby generating the valve event for the engine braking operation.
The engine brake device disclosed by Cummins is a bolt-on accessory that fits above the engine. In order to mount the engine brake, a spacer needs to be provided between the cylinder and the valve cover, thus the height, weight and cost of the engine are additionally increased. Obviously, the solution to the above problems is to integrate the components of the braking device into the existing components of the engine, such as into the rocker arm of the engine, thereby forming an integrated brake.
An integrated rocker-arm brake was disclosed by the Mack Truck Company of the United States in U.S. Pat. No. 3,786,792 in 1974. The brake piston of the brake system is positioned in a rocker-arm cylinder arranged at one end close to a push rod and is hydraulically locked in a protruding position, so as to transmit the motion of the cam to an exhaust valve (there is only one valve per cylinder in an early engine), thereby producing the engine braking operation. A conventional cam lobe and a braking cam lobe are integrated in the above cam. The brake control valve mechanism (a combination of a funnel-shaped plunger valve and a one-way ball valve) in the above brake system was widely used after its disclosure.
Another integrated rocker-arm brake was disclosed by the Jacobs Company (JVS) of the United States in U.S. Pat. No. 3,809,033 in 1974. The brake piston of the brake system is positioned in a rocker-arm cylinder arranged at one end close to a valve bridge and is movable between a non-braking position and a braking position. In the braking position, the brake piston is hydraulically locked in a protruding position, so as to transmit the motion of the cam to the valve bridge to open two exhaust valves (the engine has two valves per cylinder), thereby producing the engine braking operation. The braking system uses two separate oil passages, one for supplying oil to the brake, and the other being a conventional engine lubrication oil passage.
An integrated rocker-arm brake system for an overhead cam four-valve engine was disclosed by Sweden's Volvo Company in U.S. Pat. No. 5,564,385 in 1996, which is very similar in both structure and principle to the integrated rocker-arm brake disclosed by Jacobs Company (JVS) in U.S. Pat. No. 3,809,033 in 1974. The hydraulic brake piston is positioned in a rocker-arm cylinder arranged at one end close to the valve bride, and is movable between a non-braking position and a braking position and forms a gap in the engine valve system. Oil with a certain pressure is supplied to the brake piston by a pressure control valve to fill the gap in the rocker arm so as to form a hydraulic linkage. The engine braking system adopted the combined structure having a funnel-shaped plunger valve and a one-way ball valve, and added an overload pressure relief mechanism and an oil supply device for providing dual oil pressures via a single oil passage, wherein a low oil pressure (below the engine lubricating oil pressure) is used for the engine lubrication, and a high oil pressure (equal to the engine lubricating oil pressure) is used for the engine brake. During engine braking, the brake piston pushes the valve bridge to open the two exhaust valves simultaneously for braking operation.
Another integrated rocker-arm brake was disclosed by the Mack Truck Company of the United States in U.S. Pat. No. 6,234,143 in 2001, which is quite different from the technology disclosed in U.S. Pat. No. 3,786,792 in 1974. First of all, an Exhaust Gas Recirculation (EGR) cam lobe was added to the integrated cam formed with the conventional cam lobe and the braking cam lobe, which facilitates improving the braking power. Secondly, the engine with a single valve per cylinder is changed into an engine with dual valves per cylinder, and a valve bridge (an air valve bridge or a cross arm) was added. Further, the brake piston in the rocker-arm piston bore is moved from the push rod side to the valve bridge side, and is located above the exhaust valve (an inner valve) next to the rocker-arm shaft. During braking, the brake piston opens one exhaust valve via a braking-push-rod or by a direct action on the valve bridge. However, since only one valve is opened for braking, the valve bridge is in an inclined state and an asymmetric load will be generated on the valve bridge and the rocker arm. Furthermore, the braking valve (the inner valve) lift profile is greater than the non-braking valve (an outer valve) or the conventional valve lift profile (larger opening and later closing).
An integrated rocker-arm brake system having a valve lift reset mechanism was disclosed by Cummins Engine Company in U.S. Pat. No. 6,253,730 in 2001 to resolve the problems of the one-valve (the inner valve) braking, such as the asymmetric load and the braking valve (the inner valve) lift profile being greater than the non-braking valve (the outer valve) or the conventional valve lift profile (larger opening and later closing). The valve lift reset mechanism resets or retracts the brake piston in the rocker arm before the braking valve reaches its peak valve lift, which allows the braking valve to return to the valve seat before the start of the main valve action, such that the valve bridge returns to the horizontal position, and the rocker arm can open the braking valve and the non-braking valve evenly, thereby eliminating any asymmetric load.
However, there are a lot of problems with resetting the engine braking system before the braking valve reaching its peak valve lift. Firstly, during engine braking, both the opening time and the lift magnitude of the braking valve are very short, thus the time for resetting is very limited. Secondly, the resetting occurs when the engine braking load is close to the maximum (i.e. the top dead center of the compression stroke), thereby causing the reset valve of the valve lift reset mechanism to bear a high oil pressure or a large load. Thus, the engine brake resetting timing is essential. If the resetting occurs too early, the loss of braking valve lift is too much (causing a lower valve lift and the valve to be closed too early), which may reduces the braking performance. If the resetting occurs too late, the braking valve can not be closed before the start of the main valve action, which may results in an asymmetric load.
Tests show that the integrated rocker-arm brake cannot work properly at high engine speeds, because the resetting time is too short, the resetting height is too small, and the load or pressure on the reset valve is very high.